Impurity contamination of Si semiconductor wafers is a problem within the semiconductor industry. Of particular concern are metallic contaminants, such as iron, nickel and copper. When such impurities are present in a Si semiconductor device, the impurities degrade the characteristics and reliability of the device. As integration in semiconductor devices becomes increasingly dense, the tolerance for metallic contaminants becomes increasingly stringent.
Among the methods for decreasing metallic contamination in semiconductor wafers are methods for improving cleanliness in plants which manufacture such semiconductive devices. However, regardless of how many steps are taken to insure clean production of semiconductor devices, some degree of contamination by metals is inevitable. Accordingly, it is desirable to develop methods and structure for isolating metallic contaminants present in semiconductor wafers from devices which are ultimately formed within and upon such wafers. The act of isolating these contaminants is generally referred to as gettering, as the contaminants are gathered, or gettered, to specific areas within a semiconductor wafer.
Conventional processes for gettering metallic contaminants often focus on creating defects or damage within a semiconductor wafer in a region where gettering is sought to occur. Generally, such gettering regions are formed well below the regions of a wafer where device formation will ultimately occur and separated from such regions by an expanse of substrate. Two embodiments of such prior art methods are shown with reference to FIGS. 1 and 2. Referring to these figures, a semiconductor wafer 10 comprises a front-side surface 12 and a back-side surface 14. Front-side surface 12 is defined as a surface where device formation will ultimately occur. A damage region 16 is formed beneath front-side surface 12 and is placed deep enough within the substrate that later devices formed on front-side surface 12 are isolated from the damage region 16. Damage region 16 is typically formed by introducing impurities into the lattice of the semiconductor material of wafer 10. In FIG. 1, damage region 16 is a layer within the middle of substrate 10, while in FIG. 2, damage region 16 is a layer along back-side 14 of wafer 10. After damage region 16 is formed, wafer 10 is heated to drive metallic contaminants into the damage region.
A problem of increasing concern as semiconductor devices become increasingly smaller is substrate-background current, or diffusion current. Such diffusion current is function of device temperature and increases exponentially with temperature. Thus, if the temperature of a semiconductor wafer increases, such as typically occurs during operation of semiconductor devices, the diffusion current generally also increases. At a given temperature, more diffusion current will generally form from a defect region of a semiconductor wafer than from a region without defects. Thus, damage regions 16 tend to generate more diffusion current at a given temperature than do other regions of a semiconductor wafer 10.
The diffusion current electrons formed in damage region 16 will generally drift away from damage region 16, potentially toward front-side surface 12. Such electrons at front-side surface 12 may degrade the performance of devices that are later formed on surface 12.
For the above-described reasons, it would be desirable to develop a gettering region which could collect diffusion current electrons. Also, since hole counterparts of the diffusion current electrons can also be generated as the diffusion current electrons are generated, it would also be desirable to develop a gettering region which could collect such holes.